1. Field of the Invention
This invention relates to CD-ROM (compact-disc read-only memory) drive systems, and more particularly, to an all-digital frequency following system which is constructed entirely based on digital circuit elements for use on a high-performance CD-ROM drive system with an operating frequency of above 40 megahertz (MHz).
2. Description of Related Art
During the operation of a CD-ROM drive system, a decoder is used to receive data from a digital signal processor (DSP) and then store the received data in a memory unit, such as a dynamic random access memory (DRAM) unit. To allow the CD-ROM drive system to operate with high performance, it requires the provision of a reference clock signal of a high frequency that can be stably maintained at the desired level.
In conventional CD-ROM drive systems, the reference clock signal is usually generated by a crystal oscillator whose output frequency or a multiple thereof is used as the clock signal. Conventionally, the multiplication of the output frequency of the crystal oscillator is achieved by using a phase-lock loop (PLL) circuit. In addition, high frequency generators or analog PLL devices can be used to directly obtain the desired high frequency.
One drawback to the use of the above-mentioned analog means to generate the high-frequency reference clock signal, however, is that the circuit will be complex in structure and poor in stability. Since most of modern electronic devices are based on digital circuits, there exists, therefore, a need for an all-digital frequency following system which is constructed entirely based on digital circuit elements and capable of generating an output frequency which can be either an integral or an irrational multiple of an input frequency.